The Fletcher-Munson effect is the name given to the well-known fact that human perception of sound in the bass frequency band (frequencies less than about 500 Hz) is influenced by the volume level of the sound. In particular, the sensitivity of a human listener to bass frequencies decreases as volume level decreases, while the sensitivity to higher frequencies decreases to a lesser degree. Accordingly, music played at low volume levels is perceived as lacking bass frequencies. The Fletcher-Munson effect can be compensated for by increasing the sound energy at low frequencies relative to the sound energy at higher frequencies.
To achieve such compensation, audio filter circuits have been designed to increase the bass frequency band of an electrical audio signal (referred to herein as an audio signal) used to produce an audio output, relative to higher frequencies, as volume level decreases. Such systems are sometimes referred to as bass boost circuits.
Compensation may be provided using a manual, two-level compensation scheme, such as a conventional loudness feature on a stereo, or compensation may be dynamic. Dynamic filter circuits produce sounds with bass frequencies that are automatically controlled relative to the higher frequencies as a function of the volume of the sound produced. Such conventional audio circuits may quantify volume level based on sound pressure or based on the amplitude of an audio signal, and may increase low frequency components of the sound output.
All known dynamic bass boost circuits have had one or more drawbacks, including but not limited to excessive complexity, excessive cost and an inability to produce a high quality sound output. Accordingly, there is a need for improved dynamic bass boost apparatus and methods.